For detection and optional determination of the concentration of substances such as methane, ethane, propane, carbon monoxide, hydrogen, etc. in gas mixtures, heating gases, exhaust gases and/or the like, gas sensors based on inorganic metal oxide semiconductors (MOX) are known from the state of the art. When such gas sensors, which are designed as a porous solid, come in contact with the aforementioned substances or other reducing substances, the conductivity of the metal oxide changes at least at its surface. The change in conductivity of the metal oxide can be detected and analyzed by means of an electronic unit to detect reducing substances and optionally determine their concentration.
Such a gas sensor is known from DE 44 28 155 C2. The gas sensor comprises a substrate on which an interdigital electrode structure is created in the form of two comb-like intermeshing electrical elements that are spaced a distance apart from one another. A layer of gas-sensitive metal oxide, gallium oxide here, is applied to the interdigital electrode structure and is in contact with the same. On coming in contact with a flammable gas, the conductivity of the metal oxide changes and the gas concentration can be determined accordingly by measuring the electrical resistance between the two electrical elements of the interdigital comb-like intermeshing electrode structure spaced a distance apart from one another.
However, the changing conductivity of the gas-sensitive metal oxide is a function of temperature. Therefore in DE 44 28 155 C2 a temperature sensor is provided as a separate element on the substrate in proximity to the interdigital electrode structure. The actual temperature of the gas sensor must thus be ascertained and taken into account in analyzing the measured resistance values.
If the temperature is too low, the gas sensor known from DE 44 28 155 C2 cannot be used because an electron transition of the adsorbed molecules of the reducing substances, causing a change in conductivity, takes place in a quantity that can be analyzed reasonably only at temperatures above 450° C. in the case of gallium oxide. Gallium oxide is thus an n-type conductor at temperatures above approximately 450° C. In the range between approximately 450° C. and 700° C., gallium oxide has a strong sensitivity to reducing substances, such as methane, ethane, propane, carbon monoxide, hydrogen and the like which is a result of surface interactions. Therefore the entire gas sensor is limited to use purposes in which the gas to be monitored has a sufficiently high temperature. This is the case with exhaust gases of combustion processes, for example.
To be able to use a gas sensor based on metal oxide even at lower ambient temperatures, EP 0 464 244 B1 describes also providing an electrical element for heating on a substrate which has comb-like intermeshing electrical elements that are spaced a distance apart from one another for measuring the conductivity of the metal oxide. The electrical element for heating is embodied in the form of a meandering printed conductor, which may carry a heating current and is situated on the surface of the substrate opposite the electrical elements for measuring the conductivity of the metal oxide. It is thus possible to heat the metal oxide, which is again embodied as gallium oxide, to a temperature above 450° C.
Due to the design of the electrical elements for heating the metal oxide on one side of a substrate and for measuring a conductivity of the metal oxide on the opposite side of the substrate, this yields disadvantages with the gas sensor known from EP 0 464 244 B1. Thus, on the one hand, production is complex and expensive but on the other hand, the heating energy does not act directly on the metal oxide surrounding the electrical elements for measuring the metal oxide but instead acts through the substrate. Finally, the sensitivity of the sensor is not optimal because there is only a limited amount of metal oxide between the electrical measurements for measuring the metal oxide and the sensor is gas-sensitive on only one side.
Under typical use conditions, the sensors are exposed to high temperatures up to 1200° C., mechanical stresses in the form of vibrations and thermal stresses due to extreme temperature changes and must also be able to withstand chemical influences due to reducing and corrosive gas components over long periods of time without undergoing any significant aging, so there are weaknesses in principle with the known substrate-based sensors, which must be compensated by increased technical complexity. With the gas sensor known from EP 0 464 244 B1, passivation by means of silicic acid is necessary to prevent catalytic reactions on the open surfaces of the heating arrangement. In vaporization of the passivation layer due to the aforementioned adverse operating conditions, there are negative effects on the sensitivity of the gas sensor. Furthermore, the substrate may be excited to vibrations, and thermal stresses may lead to damage to the electrical elements on the substrate.
Finally, production of a substrate-based gas sensor such as that known from the DE 44 28 155 C2 or EP 0 464 144 B1 is technically complex and expensive because special equipment is required to form small structures on a substrate.
Although U.S. Pat. No. 3,835,529 describes a substrate-free gas sensor based on an inorganic metal oxide semiconductor, no heating element is provided in the substrate-free embodiment to heat the metal oxide. When a heating element is present, it is applied to a backing, which is a substrate that must also be heated. This leads to the disadvantages mentioned above. Furthermore, a cylindrical, gas-impermeable metal housing is obligatory with the gas sensor known from U.S. Pat. No. 3,835,529 because the metal oxide is present in a loose powder form. The gas-impermeable housing reduces the sensitivity of the sensor because gas cannot reach the sensor on all sides and instead the gas must flow through end caps to the sensor. Furthermore, heating of the gas sensor results in unfavorable inherent thermal conditions. Finally, the loose packing of the metal oxide has a negative effect on the signal stability because mechanical stresses such as vibrations interfere with the contact structure.
With the gas sensor known from DE 10 2008 055 568 A1 the electrical elements for heating are designed separately from the electrical elements for performing measurements. No electrical elements are provided here to be used jointly for heating and measuring and none are embedded spatially in metal oxide. Instead DE 10 2008 055 568 A1 discloses a classical layer structure.
With the gas sensor known from DE 198 59 998 A1 the measurement elements are arranged on a substrate. This is fundamentally problematical because there may be faulty flows. The substrate is recessed between measurement elements, and the metal oxide layer applied over the measurement elements extends into the recess. A heater designed separately from the measurement elements is uncoupled from the sensitive layer.
The gas sensor of DE 44 01 570 A1 does not have a heating element, is not sensitive on all sides and requires a third electrode which is insulated from a semiconductor layer.
Finally DE 690 18 742 T2 discloses a thermistor and a gas sensor with this thermistor wherein the heater is designed separately from the thermistor. Furthermore, the thermistor does not have porous material and the gas sensor is not sensitive on all sides.
Against this background, the object of the invention is to create a heatable gas sensor and a method for producing same according to the preamble of claim 1 and/or 6 which will permit a longer use time even under adverse use conditions while achieving an increased sensitivity and simplifying the design in a manner that also lowers manufacturing costs.